The invention relates to a method of bonding contact facings composed of cadmium-free silver/metal oxide materials which contain at least tin oxide or iron oxide as the essential metal oxide to a metallic contact carrier, in particular by brazing or welding. In addition, the invention also relates to associated contact facings.
Silver/metal oxide contact facings can be formed as individual contact pieces or as linearly extended contact profiled sections or contact strips. Such contact facings have to be electrically and mechanically bonded to metallic contact carriers. Contact facings of this type cannot always be reliably bonded to the contact carriers by the known methods of brazing or welding. Frequently, contact facings of this type are therefore provided with a layer of pure silver. This layer can be applied by various production methods: in the case of profiled sections, it is done by cladding the contact-material strips or by direct two-layer extrusion, but when manufacturing molded articles it is done by by pressing two separately introduced layers. In all cases, the application of the pure-silver layer is associated with an increased technical complexity in the manufacture. This entails an additional financial cost which results in an appreciable increase in price of such contact facings.
In the case of single-layer contact facings which are to be applied by soldering to metallic contact carriers, German Offenlegungsschrift 22 60 559 has already proposed dissolving the metal oxide component out chemically in order to achieve better soldering properties. The latter has not, however, gained acceptance in practice since only the oxides situated directly at the surface are dissolved out by the chemical removal of the metal oxide component. In the soldering process, however, silver is generally dissolved by the solder, with the result that an inadequate oxide removal results in a poor wettability and not, consequently, in a reliable soldering. From SU-A-1108522 it is known to reduce the solder side of contact pieces composed of silver/metal oxide materials by means of electrochemical processes in order to make bonding to the contact carrier possible without a separate silver layer. Finally, with JP-A-5-182558 is described the manufacture of silver-metal oxide/metal compound arrangements based on silver/cadmium oxide, strips of silver/cadmium oxide material which can contain further metal oxides as minor constituents being heated by current heating in a N.sub.2 /CO atmosphere and a selective reduction being carried out at the surface of the strip. With these means a silver/cadmium solder is formed in the subsurface region of the contact facing, which means the switching properties on the switching side get lost. Therefore, after the bonding of the contact pieces to the contact carrier, the contact side must again be oxidized to higher valency.